1. Field of Invention
This invention relates to a DC motor, especially one with a very high rotational velocity.
2. State of the Prior Art
Motors that rotate at very high speed are desirable, but there are many reasons that ultra-high speeds are difficult to attain. Most motors cannot switch the electromagnets fast enough because they use a mechanical commutator to transmit electricity from the stator to the rotor. Most also rely only on magnetic attraction. These motors suffer from inherent frictional losses and are susceptible to rapid degradation. They also produce radio frequency interference by the sparking of the commutator brushes. These devices are incapable of self-starting and require a generator to initiate the operation of the motor. The need for a generator subjects the motor to erratic behavior caused by temperature responsive zero drift. The use of a generator also complicates the circuitry of the motor.
Several brushless DC motors have been developed with varying degrees of success. The initial brushless motors used a permanently magnetized rotor and a plurality of stator windings. As a selected stator winding is energized, the rotor turns, which reduces the torque angle between the magnetic fields of the stator and of the rotor. Rotation is maintained by sequentially energizing the stator windings so that a large torque angle always exists. Stationary inductive pick-up coils determine the position of the rotor relative to the stator so that the electrical commutator can activate the desired stator winding at a rate proportional to the angular velocity of the rotor. Rotation of the permanently magnetized rotor induces the voltages within the pick-up coils. The coil arrangement is incapable of self-starting the motor without elaborate starting mechanisms.
U.S. Pat. No. 3,324,370 to Studer (1967) discloses a brushless DC motor using an electronic beam switching tube as a commutator. The electronic beam switching device includes pulse input switches that sequentially switch a plurality of outputs to succeeding outputs. A variable frequency oscillator, which responds to control signals from a pick-up coil transducer device, and a rotor position sensing device control the switches. Rotation of the magnetic rotor induces a voltage in the coil. An increase in the rotor's angular velocity marks an increase in the transducer's voltage. This results in a greater output frequency from the oscillator. The motor uses one layer of approximately ten windings, which surround a permanently magnetized rotor. As electron beam switching tube activates each winding sequentially to maintain a torque angle between the windings and the rotor. The ten stator pole motor is said to be able to produce a rotation of about 8000 rpm.
U.S. Pat. No. 3,924,167 to Clark (1975) discloses a two pole, permanent magnet rotor positioned within a stationary armature. The stationary armature has three symmetrically positioned sets of windings, each set of windings occupying 120.degree. of the armature surface. The motor also includes a self-commutation means which produces three switching signals. The on/off variations in the signals indicate the passage of a reference radius on the rotor by each of the rotor's three equiangularly spaced reference points. The three switching signals energize the three sets of windings in sequence to allow the continuous operation of the motor. The use of only three sets of windings requires each set to carry a relatively large magnetic field to produce a force large enough to rotate the rotor.
The stepper motor in Jahelka, U.S. Pat. No. 4,158,800 (1979) uses a closed loop feedback system that includes a coded disk driven by the motor and decoding means to derive signals from the disk. The signals include direction signals, select signals and speed signals. The velocity of the motor varies relative to the lead angle created between the energized motor windings and the motor shaft position.
U.S. Pat. No. 4,507,590 to Miyazaki (1985) discloses a brushless DC motor with a single position detector. This design includes drive coils on the stator a magnetic rotor with alternating poles, a position detector and a drive circuit that supplies current to the drive coils. The position detector reads information from the rotor, which a code converter converts to digital phase signals. A digital-to-analog converter changes the output to an analog signal, which is amplified and applied to the coils. This device requires a position detector for each phase coil used. The plurality of position detectors needed complicates the circuitry of the motor and sets a relatively low maximum number of phase coils which can be used in the motor's design. The low number of phase coils restricts the power derived from the motor. This design also tends to produce harmonic components that can interfere with the operation of the motor.
The foregoing devices represent improvements in the design of brushless DC motors. However, these devices are incapable of efficiently and effectively rotating the rotor within the stationay stator. The ability to expand these devices is also very limited. The circuitry required to increase the number of windings in each of these designs renders the expansion of these motors impractical.